The Lady Cassandra O'Brien Dog Delta Seventeen is the last "pure" human, born (as a boy) on Earth to two human parents (father from Texas, mother from the Arctic desert) who were also the last two people to be buried on the Earth. As a child, s/he lived in a house built into the side of the Los Angeles crevasse. Cassandra lives to be over 2,000 years old, during which time she has at least five husbands... and 708 operations. Over the course of these operations, Cassandra has her skeleton and organs removed, leaving her a sheet of skin and blood vessels stretched inside a frame, at the base of which rests a tank containing her brain. Once she has her chin removed, she considers herself to be thin and dainty; Rose calls her a "bitchy trampoline". Her 709th operation would have bleached her blood. As a result of all her operations, Cassandra's skin is very delicate and prone to drying out; she therefore requires continual moisturization, and is accompanied by a pair of attendants armed with misting hoses (which can also spray acid, when necessary). Despite her radical bodily modifications, Cassandra is dismissive and disdainful of partial-humans who have been interbred with alien species.
Cassandra's continual flattening operations have put a severe drain on her finances; in order to recoup some of the costs, she buys shares in the rival companies of several highly wealthy citizens of the universe, then ensures that they are all invited to Platform One to observe the death of the Earth. Once there, she sets her plan in motion: her thought-controlled androids will pose as one of the delegations, the Adherents of the Repeated Meme from Financial Family Seven, and in accordance with the custom of giving a "gift of peace" to the attendees, they will distribute metal spheres containing Cassandra's specially-programmed metal spiders. When the delegates have carried their spheres into their chambers, and thus past the codewalls, the spiders will be able to infiltrate the station, sabotage its systems and control the mainframe, to create a hostage situation. As a clear victim of the situation, Cassandra would never have come under suspicion, and would have received a huge compensation.
Once her plan is discovered by the Doctor, she switches to her backup plan: an ostrich egg which she brought aboard the station hides a power feed for her teleporter, which transports her off the station as the spiders destroy the Platform's safety systems. When the station is destroyed along with the Earth, her rivals' stocks will plummet and hers will rise. However, the Doctor is able to trip the manual reset switch and restore the Platform's shields, then reverses the teleporter and brings Cassandra back aboard the station. Separated from her attendants, and with the elevated temperatures (caused by the sun's rays breaching the damaged shields) rapidly drying out her skin, Cassandra is helpless and ultimately bursts apart as her skin dries and tears on the metal frame.
